The search for Nanobubbles by using specular and off-specular Neutron Reflectometry

TL;DR

This study uses neutron reflectometry techniques to investigate the presence of nanobubbles at a hydrophobic silicon/water interface, finding no evidence of nanobubbles at detectable levels after saturation with nitrogen gas.

Contribution

The paper demonstrates that specular and off-specular neutron reflectometry, combined with GISANS, can effectively assess nanobubble presence and sets limits on their surface coverage at the interface.

Findings

No significant nanobubbles detected after nitrogen saturation.

Specular reflectometry suggests a depletion layer at the interface.

Off-specular measurements show no change with nitrogen saturation.

Abstract

We apply specular and off-specular neutron reflection at the hydrophobic silicon/water interface to check for evidence of nanoscopic air bubbles whose presence is claimed after an ad hoc procedure of solvent exchange. Nanobubbles and/or a depletion layer at the hydrophobic/water interface have long been discussed and generated a plethora of controversial scientific results. By combining neutron reflectometry (NR), off-specular reflectometry (OSS) and grazing incidence small angle neutron scattering (GISANS), we studied the interface between hydrophobized silicon and heavy water before and after saturation with nitrogen gas. Our specular reflectometry results can be interpreted by assuming a sub-molecular sized depletion layer and the off-specular measurements show no change with nitrogen super saturated water. This picture is consistent with the assumption that, following the solvent exchange, no additional nanobubbles are introduced at significant concentrations (if present at all). Furthermore, we discuss the results, in terms of the maximum surface coverage of nanobubbles that could be present on the hydrophobic surface compatibly with the sensitivity limit of these techniques.